A human liver cDNA library has been established in E. coli strain RR-I and HB-101 using the plasmid pBR322 as cloning vector. A 17 base long synthetic oligonucleotide based on residues 108-113 of apolipoprotein A-I and a 26 baselong apoA-I cDNA obtained by extending the 17 base long synthetic primer were used as hybridization probes to select for apoA-I sequence bearing recombinant plasmids. Nucleic acid ssequence analysis of the apoA-I cDNA clones revealed that apoA-I mRNA codes for a precursor apolipoprotein, preproapoA-I, which contains a 24 amino acid extension on the NH2-terminal end of the mature plasma apoA-I. Eighteen amino acids are contained within the hydrophobic prepeptide (Met-lys-Ala-Ala-Val-leu-Thr-leu-Ala-Val-leu-Phe-leu-Thr-Gly-Ser-Gln-Ala) followed by a 6 amino acid propeptide (Arg-His-Phe-Trp-Gln-apoA-I) which is identical to the NH2-terminal amino acid sequence of plasma and lymph apoA-I1 isoform, thus confirming that the apoA-I1 isoform is the proapoA-I encoded by apoA-I mRNA. The complete nucleic acid sequence of human preproapoA-I mRNA has also been determined for the first time. Several structural variants of apoA-I have been recognized which are associated with low levels of plasma HDL including apoA-ITangier, apoA-Milano, and apoAIMarburg. A restriction endonuclease map of the apoA-I genome has been established by Southern blotting analysis of chromosomal DNA obtained from the peripheral blood of normal volunteers. Our results indicated that major apoA-I gene is contained in a 2.1 kb Pst-I fragment and there is no gross difference in structural organization between normal apoA-I and the Tangier disease apoA-I gene. Current studies are underway to identify the defect in the apoA-I gene in several different dyslipoproteinemias characterized by low levels of HDL.